1. Field of the Invention
The present invention relates to a cross-coil type analog indicating instrument provided with a pair of coils arranged in quadrature which drives a pointer of the instrument according to a composite electromagnetic force generated by the coils.
2. Description of the Related Art
There are related prior art devices disclosed in U.S. Pat. No. 5,121,347 and U.S. Pat. No. 5,313,155 which were patented to the assignee of this application. In the above-mentioned prior art devices, a car-speed signal generated by a car-speed signal sensor is converted into a voltage signal, and two different phase sawtooth currents are generated according to the voltage signal as exemplified in FIG. 3A and FIG. 3B of this application. The saw tooth currents are then converted by respective I-V converters into triangular wave voltages as exemplified in FIG. 4A and FIG. 4B. Thereafter, the wave slope of the triangular voltages are changed by first and second stage function generators to provide approximated sinusoidal function voltages. Subsequently, a rectilinearly approximated sine wave current and a rectilinearly approximated cosine wave current are formed based on the function voltages and supplied to the cross-coils to drive a pointer of an analog indicating instrument.
In the above mentioned prior art device, when the function voltages are formed by changing the slope of the triangular voltage waves, it is necessary to provide two different phase triangular wave voltages one of which turns its direction at the zero volt point where an input analog voltage VD (an output voltage of F-V converter 50 shown in FIG. 1) is equal to 1 V ( which corresponds to the pointer rotation angle 180.degree.) as exemplified in FIG. 4A, and the other of which turns its direction at zero volt point when VD=0.5 V ( which corresponds to 90.degree.) and VD=1.5 V ( which corresponds to 270.degree.) as exemplified in FIG. 4B.
However, in the above prior art devices, such approximated sinusoidal function voltages may not turn the direction at the zero volt point on the prescribed phase angles due to variations of parts and components used in the device, and an approximated sine wave current or an approximated cosine wave current applied to the cross-coils become distorted. As a result, the cross-coils 11 and 12 generate a distorted rotating magnetic force which is the composite vector of these two currents, thereby resulting in that the pointer of the indicating instrument may jump or stagnate on a dial.